1. Field Of The Invention
The invention relates to an image display unit for displaying pixels of an image in a plurality of periods, called sub-fields, on a display device, which is capable of generating, in each of the sub-fields, a respective illumination level, and which comprises computing means to perform operations on sub-fields for motion compensation.
The invention further relates to an image display apparatus comprising:
receiving means for receiving a signal representing the image;
a display device for displaying the image; and
an image display unit for displaying pixels of an image in a plurality of periods, called sub-fields, on a display device, which is capable of generating, in each of the sub-fields, a respective illumination level, and which comprises computing means to perform operations on sub-fields for motion compensation.
The invention further relates to a method of displaying pixels of an image in a plurality of periods, called sub-fields, on a display device, which is capable of generating, in each of the sub-fields, a respective illumination level, comprising a motion compensation step on sub-fields.
2. Description Of The Related Art
An image display unit of the kind described in the opening paragraph, is known from the article “Motion Compensation in Plasma Displays”, Proceedings of The Fifth International Display Workshops, IDW 1998, pages 543-546. In this article, it is described that on current plasma display panels, disturbing motion artifacts are perceived as dynamic false colors or pseudo-color appearances due to sub-field illumination scaling. The article summarizes that many solutions have been proposed to reduce these artifacts, for instance, changing the order of displayed sub-fields; applying bit or sub-field splitting to divide major sub-fields; and scattering false colors by multiple sub-fields with equal illumination levels in which the same illumination levels are generated by different combinations of these sub-fields. None of these methods eliminate the basic cause of the problem. They only try to mask the effect in areas with a small spatial luminance gradient. The article provides an analysis of the problem of motion artifacts. The motion artifact itself is due to the tracking of motion by the observer's eyes, and the time difference between the various sub-fields that are displayed. Due to the tracking of motion, various sub-fields that ought to be perceived at one position of the eye, are perceived at different positions, and the different sub-fields from nearby pixels are accumulated at the same position on the retina and contribute to the illumination level that is perceived instead of the intended one. When an observer focuses on a moving object, he will start tracking the movement. The object is kept at exactly one position on the retina. Due to the speed, {right arrow over (v)}=(vx,vy), of this object, a certain distance is traveled while following this object for a certain period. When this same object is observed on a plasma display panel, the positions seen are determined by the starting position, {right arrow over (x)}=(x,y), of this object and the time difference, Δtn, of the observed sub-field, SFn ({right arrow over (x)}). The observed luminance at this position, L({right arrow over (x)}), when this motion is being tracked by the observer, is determined by the observed positions on the screen. This depends on whether or not sub-field SFn({right arrow over (x)}) at position {right arrow over (x)}, is on, and on the illumination level Wn of this sub-field:                               L          ⁡                      (                          x              →                        )                          =                              ∑                          n              =              1                        N                    ⁢                                           ⁢                                                    SF                n                            ⁡                              (                                                      x                    →                                    +                                                                                    v                        →                                            ·                      Δ                                        ⁢                                                                                   ⁢                                          t                      n                                                                      )                                      ·                          W              n                                                          (        1        )            with Δtn=tn−t0, the time difference between sub-field n and the reference time t0, and the speed {right arrow over (v)} expressed in pixels per field period.
The article also provides a solution for the problem of motion artifacts, i.e., motion compensation. Motion compensation can reduce dynamic false contouring and pseudo-color appearance without reduction in sharpness or loss of detail. Motion compensation attempts to position the sub-field values of that one pixel that is being tracked exactly at the positions on the display panel that are observed at the time the sub-fields are generated and at the position that is seen. It can be inferred from Equation 1 that a spatial offset of {right arrow over (d)}n=(dxn,dyn), must be given to each sub-field SFn({right arrow over (x)}), to be able to place these sub-fields at the correct positions, resulting in a luminance:                               L          ⁡                      (                          x              →                        )                          =                              ∑                          n              =              1                        N                    ⁢                                           ⁢                                                    SF                n                            ⁡                              (                                                      x                    →                                    +                                                                                    v                        →                                            ·                      Δ                                        ⁢                                                                                   ⁢                                          t                      n                                                        -                                                            d                      →                                        n                                                  )                                      ·                          W              n                                                          (        2        )            In order to avoid artifacts, {right arrow over (d)}n is chosen to be:{right arrow over (d)}n={right arrow over (v)}·Δtn−{right arrow over (d)}ne  (3)with {right arrow over (d)}n =(dxn,dyn) the displacement in the horizontal and the vertical directions, which is rounded to integer values, and {right arrow over (d)}ne=(dxne,dyne) the rounding error. A sub-field must be displayed over an integer number of pixels, because no parts of a pixel can be switched on or off. The spatial offset {right arrow over (d)}n=(dxn,dyn) for each sub-field can be calculated by making use of a motion vector {overscore (m)}x,y of the corresponding pixel:                                           d            →                    n                =                                            t              n                                      T              field                                ·                                    m              _                                      x              ,              y                                                          (        4        )            where Tfield denotes the time of one field period.
The number of operations required for achieving motion compensated images, i.e., spatially corrected sub-fields, is relatively high. The operations include memory accesses and processor calculations to determine the spatially corrected sub-fields. Especially in the case of a programmable processor architecture, this relatively high number of operations requires large computer resources, resulting in relatively high costs.